It was in Chapter 20 when Atticus mentioned the "crime"--however, he called it a "code" rather than a crime that was broken.

"She has committed no crime, she has merely broken a rigid and time-honored code of our society, a code so severe that whoever breaks it is hounded from our midst as unfit to live with."

He continues on in Chapter 20 explaining that she knew what she was doing. She didn't plan it from the beginning, but by accusing Tom, she was getting herself out of trouble with her father (and society--although she's already an outcast from society).

"She knew full well the enormity of her offense, but because her desires were stronger than the code she was breaking, she persisted in breaking it."